TL431 is a three-terminal shunt regulator chip having popular applications, such as current limit, current source, constant current sink, fixed/adjustable reference generator, constant voltage supply, constant current supply, operational amplifier, boot-strap circuits, low-power bias supply and so on. The symbolic circuit of a TL431 is shown in the left part of FIG. 1, whose right part is shown the internal block diagram thereof, in which a shunt regulator 10 has a cathode 12, an anode 14 and a reference electrode 16, between the cathode 12 and the anode 14 is a bipolar junction transistor (BJT) Q, and an error amplifier 18 determines the base bias of the BJT Q according to the difference between the voltage Vref of the reference electrode 16 and an internal reference voltage VI, to control the current Ik flowing from the cathode 12 to the anode 14. Referring to FIGS. 1 and 2, when the voltage Vref is smaller than the voltage VI, the BJT Q is cut off, and the current Ik is almost zero; when the voltage Vref is larger than the voltage VI, the current Ik increases with the difference between the voltages Vref and VI; and when the voltage Vref is larger than a certain value VM, the current Ik stops increasing, and maintains at a maximum value. It can be known from FIG. 2, when the shunt regulator 10 operates with a relatively large voltage Vref, the current Ik is relatively large, and consequently the power loss is relatively large. Moreover, in some applications, the relatively large current Ik will lead to a relatively large power loss in other circuits.
Green AC/DC is a trend for earth environment protection especially for no load power loss since un-used adapter may always be hung on without removing. There are some industrial standards to specify the no load power loss, such as EU and CEC. 100 mW is famous now for industrial application, but 50 mW, 30 mW development is on going. The TL431 is frequently adopted in flyback converter control system, as shown in FIG. 3, in which a transformer 20 has its primary coil Np connected between a power input node Vin and a power switch Mp, a flyback controller 22 provides a pulse width modulation (PWM) signal Spwm to switch the power switch Mp, for a secondary coil Ns of the transformer 20 to generate a current to charge a capacitor Co through a diode Do, to thereby generate an output voltage Vo, a voltage divider 24 divides the output voltage Vo to generate a feedback voltage Vfb for the reference electrode 16 of the shunt regulator 10, the shunt regulator 10 and a photocoupler 26 establish an isolated feedback circuit to determine the forward current IF of the photocoupler 26 according to the feedback voltage Vfb, to thereby control a feedback current Icomp, and the flyback controller 22 modulates the duty cycle of the PWM signal Spwm according to the voltage Vcs of a current sense resistor Rcs and the feedback current Icomp, to control the power delivery of the transformer 20 from the primary side to the secondary side, to thereby regulate the output voltage Vo. As shown in this flyback converter, the TL431 scheme consumes much power, even at no load of the flyback converter. U.S. Patent Application Publication No. 2008/0037296 discloses a green-mode flyback PWM apparatus to reduce the switching frequency of the power switch MP for light load in order to reduce the power loss of a converter. When the flyback converter operates in the green mode, referring to FIGS. 3 and 4, whenever the output voltage Vo drops to a threshold Vo_L, the PWM signal Spwm is provided to switch the power switch Mp to increase the output voltage Vo; whenever the output voltage Vo increases to a threshold Vo_H, the flyback controller 22 stops providing the PWM signal Spwm and thus the output voltage Vo decreases gradually until it reaches the threshold Vo_L again. Since the power switch Mp is not switched during the period where the output voltage Vo drops from Vo_H to Vo_L, the power loss is reduced. However, in the green mode, referring to FIGS. 2-4, the feedback voltage Vfb varies between Vfb_L and Vfb_H, and thus the shunt regulator 10 draws a relatively large current IF from the power output node Vo, causing the output voltage Vo to decrease faster, thereby resulting in more times of switching of the power switch Mp. For example, as shown by the waveforms 28 and 30 of FIG. 4, the larger the current IF is, the faster the output voltage Vo decreases, and thus the faster the Vo_L is reached, causing earlier recovery of the PWM signal Spwm to switch the power switch Mp. In other words, the period where the power switch Mp becomes silent is shortened, and the switching times increase.